Roof cleaning cost

Cleaning a roof costs $0.15 to $0.60 per square foot for a soft wash, so a 2,000 square foot roof is $300 to $1,200 with most jobs near $500. Manual moss removal costs more, $0.60 to $1.00 a foot, because it is done by hand.

The black streaks on a north-facing slope are not dirt. They are a cyanobacterium, Gloeocapsa magma, which feeds on the limestone filler in asphalt shingles. It is cosmetic for years and then it is not.

What it costs

Steep roofs, two stories, and difficult access all push toward the top of the range.

Never pressure wash an asphalt roof

This is the one rule.

A pressure washer strips the mineral granules off asphalt shingles. Those granules are the shingle's sunscreen and most of its weight; without them the asphalt underneath is exposed to ultraviolet light and the roof ages in a fraction of the time. The damage is permanent and it voids most manufacturer warranties.

A driveway contractor with a pressure washer who offers to do the roof while they are there is offering to take ten years off it for $200.

Soft washing is the correct method: a low-pressure application of a detergent, usually sodium hypochlorite based, left to dwell and then rinsed. It kills the algae rather than blasting it off. The streaks fade over the following weeks as rain carries the dead organism away.

Moss is a different problem

Algae is a stain. Moss is a plant with a root structure that lifts the edges of shingles and holds water against them. That is structural rather than cosmetic, and it is why moss removal costs more.

It is removed by hand with a soft brush, working downward so shingles are not lifted, and it is slow. Anyone offering to scrape moss off quickly is lifting shingles.

Moss grows where the roof stays damp: north slopes, under overhanging trees, in the Pacific Northwest and the humid southeast. Cutting back the branches that shade the roof does more than any treatment.

The permanent fix is a metal strip

Zinc or copper strips fitted at the ridge cost $150 to $400 and work by chemistry: every time it rains, a small quantity of metal ions washes down the roof and suppresses algae and moss growth.

The band under an existing metal chimney flashing is usually visibly clean while the rest of the roof is streaked. That is the mechanism, and it has been visible on roofs for a century.

It prevents rather than cures, so it goes on after a clean. On a roof in a humid region it is the best value in this article.

Is it worth doing at all

For appearance, yes, and particularly before selling: a streaked roof reads as an old roof to a buyer even when it is not, and a $500 clean can change the conversation about a $15,000 replacement.

For roof life, moss yes, algae marginally. The algae feeds on filler rather than structure, and the practical damage is slow. The claims that streaking halves a roof's life are made by companies selling cleaning.

Not worth doing on a roof with two or three years left. Clean it and it is still a roof with two or three years left.

Choosing someone

• Confirm soft wash, not pressure, and ask what pressure they use
• Ask how plants below the eaves are protected — sodium hypochlorite kills landscaping, and rinsing before and after is the standard precaution
• Ask whether the price includes gutter cleaning, since the debris ends up there
• Check they are insured for work at height

Prices vary widely for the same work. Three quotes is worth the effort on a job this cheap because the range is genuinely large.

Frequently asked questions

How often should a roof be cleaned?

Every two to five years in a humid or shaded location, and not at all in a dry sunny one. Cleaning a roof that is not dirty achieves nothing.

Will the streaks come back?

Yes, over three to five years, unless something changes the conditions. Zinc strips or trimming the trees that shade the roof are what change them.

Can I do it myself?

Chemically, from a ladder, on a single-story roof, with care about the plants below and about what you are standing on. Walking a wet, chemically treated roof is how people fall. The saving is a few hundred dollars.

Do bleach-based cleaners damage shingles?

Applied at the right dilution and rinsed, no; the industry standard method uses them. What damages shingles is pressure, and scraping.

Are algae-resistant shingles worth it?

Yes, in humid regions. Copper granules mixed into the surface suppress growth for the warranty period, cost a few dollars a square more, and remove this whole subject from your life.